Giant Bacteria Inhabit Fish Guts

by Yarek Hrywna

In the weird and wonderful world of bacteria, it sometimes seems
best to expect the unexpected. Take their size, for instance. Most
bacteria are only a few micrometers, or millionths of a meter, long.
It might take several thousand of them laid end-to-end to span across
a penny, and still be too thin to see with the naked eye.

But in the warm waters of the Red Sea, and off the coast of
Australia, the largest bacteria ever seen have been discovered in the
guts of a fish. Epulopiscium fischelsoni is a bacterium of
mammoth proportions. It can sometimes grow as large five hundred
micrometers, or as about the size of the period at the end of this
sentence, which is a remarkable size for bacteria.

"Epulos" as they are more commonly called, have been found in the
intestines of several kinds of surgeonfish. These surgeonfish live in
and around coral reefs, feeding on the algae and plants that grow
there. When researchers studying the fish looked into their guts,
they found tens of thousands of these bacteria there.

Their discovery came as a surprise, and their size even
more so. In fact, for several years after their discovery,
they were believed to be protists - a completely unrelated
group of microbes that are evolutionarily closer to humans.
Research was slow because unlike some other bacteria, epulos
could not be grown in the laboratory. Finally, the analysis
of a portion of their DNA revealed that they are indeed
bacteria, and not protists as previously thought.

However, the large size of the epulos was still unexpected. The
small size of most bacteria is owed to their limited abilities. Since
they have few ways to transport nutrients across their cell
membranes, they rely on diffusion to move food into their cells, and
wastes out of them. This process of diffusion is limited by the
surface area of a cell, which is the space that a cell's surface
would occupy if it were stretched out flat. As a cell gets bigger,
both its volume and surface area increase, but its surface area
increases more slowly than its volume. Above a certain size, there is
not enough surface area to absorb all of the nutrients that the
increasing cell volume needs. So, the limit of a bacterium's size is
related to the proportion of its surface area to its volume.

One can imagine a bacterium as a paper bag. Each bag can hold a
certain amount (its volume), while it is made up of only a certain
quantity of paper (its surface area). While a large bag might hold
the same amount as several small bags, if cut up and laid out flat, a
large bag would take up much less paper than the many small bags.
This is the same case with bacteria, and it is this balance of
surface area and volume that keeps most bacteria so small.

So how do epulos get so big? Detailed microscopic analysis
revealed the answer. When the bacteria were shown in cross-section,
it was apparent that the cell membrane didn't stretch smoothly around
the cell. Instead, it was convoluted and wrinkled, with many pockets
and folds. These wrinkles and folds dramatically increase the surface
area of the cell membrane. So, spread out, the cell membrane would
occupy much more space than simply the surface of the cell, but the
volume it contains is still small (but very large for a bacterium).
This would be the same as wadding up a paper bag into a ball. It
still has the same surface area if spread out, but now it has a much
smaller volume. It is this increase in surface area, while
maintaining a large volume, that keeps the biggest bacterium in the
world alive.